Most of the telephone sets presently in use are analog in nature and serve to transmit and receive analog signals along a pair of signal lines, referred to as "tip" and "ring." By contrast, much of the equipment now employed to transmit and switch telecommunications traffic is digital in nature, thus, such equipment receives and transmits blocks of digital information. In practice, an individual analog voice terminal loop (which may be connected to one or more analog phone sets) is connected to a digital telephone switch or transmission system by way of a telephone line card.
A typical telephone line card has at least one line interface circuit that has analog interface to which the tip and ring lines of the analog loop are connected. In addition, each line interface circuit has a digital interface for connection to a digital telephone switch or transmission system. Each line interface circuit typically comprises one Analog-To-Digital (A/D) converter and one Digital-to-Analog (D/A) converter. The A/D converter converts analog input signals, received at the analog interface, to digital signals for output at the digital interface. The D/A converter converts digital input signals, received at the digital interface, to analog signals for output at the analog interface. In addition to the A/D and D/A converters, a typical line interface circuit may also include one or more amplifiers, as well as a balance circuit for canceling signals: that are reflected into the line card from the analog interface when that interface is properly terminated.
For the each line interface circuit to operate correctly, the A/D and D/A converters, as well as the amplifiers and balance circuit, must all work properly. In the past, each line interface circuit on a line card was tested to verify its operability by applying a digital value to the digital interface of the line interface circuit and then measuring, via an analog instrument, the analog signal present at the analog interface. The signal at the analog interface is then compared to a reference signal representing the proper operation of the card (i.e., operation of the line card within specified tolerances). Any fault within the D/A converter, or along the signal path between that converter and the analog interface, will manifest itself as a variation in the measured analog response signal.
The A/D converter, and the signal path between that converter and the digital interface, were tested by applying an analog test signal from an analog signal generator to the analog interface. The analog test signal was converted, via the A/D converter, into a digital signal that appears at the digital interface of the line interface circuit. The resultant digital signal was processed, and then compared to a reference value representing the proper operation of the line card. Any fault in the D/A converter, or in the signal path between the analog interface and that converter, manifested itself in terms of the digital response value appearing at the digital interface.
The above-described testing method, although effective, nonetheless incurred the disadvantage that an analog test instrument was needed for analyzing the analog response signals appearing at the analog interface during testing of the each line interface circuit D/A converter and its associated transmission path. In addition, an analog signal generator was required to test the A/D converter and its associated signal path. In practice, a typical line card may contain as many as eight, or even sixteen, separate line interface circuits, necessitating a large number of test instruments and signal generators to test all, or a majority, of the line interface circuits at once. Providing the required number of test instruments and signal generators to test individual line interface circuits on a single card in an expeditious fashion has required a large capital expense. Note that it may be possible to share the analog test instrument and analog signal generator among several line interface circuits at the expense of increasing the overall test time which is undesirable.
Thus, there is a need for a cost-effective method for expeditiously testing a line card, or other type of telecommunications equipment, containing a relatively large number of line interface circuits.